A synchronously pumped optical parametric oscillation laser (OPO) owns a plurality of advantages including a wide tuning range, a compact structure and more, it may also generate a high power ultrashort pulse laser at a high repetition rate (100 MHz). A mid-infrared femtosecond OPO using a near-infrared femtosecond laser as a pumping source is a more practical method to obtain a mid-infrared ultrashort pulse, especially to obtain a mid-infrared optical frequency comb in current technologies.
However, a technical problem hampering a further development of the said mid-infrared femtosecond OPO is how to eliminate group velocity mismatching (GVM) between a plurality of interacting pump waves, signal waves and idler waves during the optical parametric oscillation process, and a plurality of consequent problems include a limited conversion efficiency and a narrowing gain bandwidth. A plurality of detrimental impacts caused by the GVM are mainly reflected in two aspects: in time domain, the pump wave and the signal wave would walk away from each other during the oscillation process, which limits the final conversion efficiency in direct; in spectrum domain, the existing of GVM may decrease the gain bandwidth of OPO, and thus extend a pulse duration, which is extremely unfavorable in obtaining the mid-infrared femtosecond laser with an ultrashort pulse duration.
In order to eliminate the detrimental impacts on optical parametric process caused by GVM, there is a plurality of existing solutions, including mainly: employing a non-collinear phase matching configuration, inserting a dispersion-managed device (such as a chirped mirror), letting the OPO operate at nearly degeneracy and else. However, dispersion-managed device s can not solve the problem of gain bandwidth narrowing caused by GVM; adopting the non-collinear phase matching method, may significantly improve the phase matching bandwidth of optical parametric process, however, it may also bring in hardly offset angular dispersion to the output laser, which is not beneficial for any subsequent usages; letting the OPO system operate at nearly degeneracy is able to make the signal wave and idler wave satisfy group velocity matching intrinsically, however, subject to a limited number of pump sources, only a few laser wavelengths can be obtained via this specific mode. Additionally, an aperiodically poled lithium niobate (APPLN) crystal with a chirped grating profile is proposed. All of the signal wavelength components of interest can satisfy phase matching condition in certain areas of the crystal, ascribed to the changing grating periods along the propagation direction. However, this specific method has equivalently shortened effective length of the nonlinear crystal, and also brought in a plurality of difficulties to both designing and machining of the crystal.
Therefore, the prior art needs to be further improved and developed.